Revision as of 21:07, 29 June 2011

Introduction to the Plasmoid JavaScript API

This document provides an overview/reference of the Simplified JavaScript API for Plasmoids. The "Simplified" refers to the fact that it isn't a full binding to all of Qt or KDE's libraries, but a highly focused set of bindings designed to make writing Plasmoids fast and easy, while remaining powerful.

The API in this documentation covers the JavaScript API as it appears in the KDE Software Compilation as of version 4.4 and higher. Changes between versions are noted in the documentation, including when new API functionality was introduced. This API ships as part of the KDE Base Runtime package, so can be relied on being there by any application that is Powered By KDE.

Examples and Tutorials

Tutorials can be found in the Plasma tutorials area here on Techbase and full, functioning examples can be found in the KDE Examples repository in the plasma/javascript folder.

Using QML

Starting with the KDE Platform 4.7 release, the Plasma team recommends writing your Plasmoids with QtQuick technologies with the Plasma QML integration API.

The QML API uses the same API as the Simplified JavaScript API with the exception of User Interface Elements, Animations and Events which are provided instead by QML itself.

The benefits of writing your Plasmoid with QML can include faster development with more sophisticated results visually and in its usability as well as much better performance when run in a QML-only Plasma shell such as Plasma Active.

What Is A Simplified JavaScript Plasmoid?

This document describes the native Plasma API available to Simplified JavaScript Plasmoids. What makes them "Simplified" is that they do not have access to the entire C++ API in the Plasma, KDE and Qt libraries (let alone things lower in the API stack). This helps ensure that these Plasmoids are more likely to work properly between releases as changes in underlying API don't affect them as well as allowing Plasma to offer stronger security guarantees around them.

To denote that this Plasmoid is a Simplified JavaScript widget, ensure that in the metadata.desktop file there is this line:

X-Plasma-API=javascript

What follows is a description of the runtime environment available to a Simplified JavaScript Plasmoid.

QtScript

The Simplified JavaScript API is powered by Qt's QtScript system which provides access to a full featured ECMA Script interpreter. If it works in ECMA Script, it will work in a Simplified JavaScript Plasmoid. As an interesting implementation note, QtScript uses the high performance ECMA Script interpreter from WebKit and shares this code with QtWebKit.

On top of the ECMA Script language, QtScript provides Qt integration features. Probably the most useful one in this context is the use of signals and slots which is Qt's callback mechanism. Signals may be emitted in QtScript by calling the signal method in question, a signal can be connected to a slot by using the connect() method (and disconnected with disconnect()) and any function defined in the Plasmoid may be used as a slot. For example:

function onClick()
{
    print("We got clicked!")
}

function onFirstClick()
{
    print("First click!")
    button.clicked.disconnect(onFirstClick)
}

button = new PushButton
button.clicked.connect(onClick)
button.clicked.connect(onFirstClick)
button.clicked()

This will print out:

We got clicked!
First click!

on the console when the Plasmoid starts, and the "We got clicked!" again whenever the button is clicked by the user.

The object that emitted the signal that caused a slot to be called can be retrieved using the QObject sender read-only property of the global plasmoid object.